Mis juhtub, kui sync.WaitGroup loendur muutub negatiivseks, kuna Done kutsed on rohkem kui Add?

If the counter of sync.WaitGroup in Go becomes negative because calls to Done() are more than Add(), a panic will occur at runtime.

WaitGroup tracks the number of expected goroutines using a counter. When calling Add(n), the counter increases by n, and with Done() — decreases by 1. If the counter becomes less than zero, it is considered a usage error.

Example of an error:

var wg sync.WaitGroup
wg.Add(1)
wg.Done()
wg.Done() // extra call, will cause panic

The panic occurs with the message:

panic: sync: negative WaitGroup counter

Therefore, it is important to strictly maintain a balance between Add and Done to avoid this error.